1. Field of the Invention
The present invention relates to a semiconductor composite apparatus such as a light emitting diode (LED), a print head that uses such a semiconductor composite apparatus, and an image forming apparatus.
2. Description of the Related Art
For example, Japanese Patent (Kokai) Publication No. 10-63807 discloses a conventional semiconductor composite apparatus in which light emitting elements and their driver circuits and semiconductor integrated circuits are fabricated. This semiconductor composite apparatus is a card type information-controlling apparatus that includes light-emitting/light-receiving elements and associated electronics on a substrate. The light-emitting/light-receiving elements are formed of semiconductor thin films, and convert electrical signals into light signals and light signals into electrical signals. The electronics includes semiconductor integrated circuit elements, capacitors, and inductors, and communicates display information with the light-emitting/light-receiving elements.
Among recent optical printheads used as a light source for an electrophotographic printer is a semiconductor composite apparatus (“Design of Light Printer” pp 114-116 (under editorship of Mukita Yoshihiro, published by TRICEPS). Light emitting diode (LED) array chips and their drive circuit chips are mounted on a printed circuit board.
The above reference “Design of a Light Printer” discloses a conventional LED unit that employs light emitting diodes (LEDs) The LED unit includes a plurality of LED chips in which a plurality of LEDs are arranged, driving integrated circuits (driving ICs) that drive the LED chips, and a unit substrate on which the LED chips and driving ICs are mounted. The LED chips are electrically connected to the driving ICs by wire bonding. The LED chips and driving ICs have a thickness of approximately 300 μm, and are die-bonded to the unit substrate by a die-bond paste.
The aforementioned conventional semiconductor composite apparatuses suffer from the following drawbacks.
For example, because the LED chips and driving ICs are connected by wire-bonding, the LED chips and driving ICs are required to have large pads. The bonding pads are, for example, 100 μm×100 μm, much larger than the light emitting regions. In other words, the pads that do not directly contribute to emission of light occupy a larger area than regions that directly contribute to emission of light. This is uneconomical in terms of efficient utilization of material.
Wire bonding is done by applying a predetermined force on the wire and the pad. Therefore, the LED chips and driving ICs need to be as thick as several hundred microns, e.g., 300 μm. Wire bonding process also requires bonding pads to be thick. Thus, the thickness of the chips cannot be reduced any further as long as the chips are mounted on the unit substrate by the processes of dicing, picking-up, and bonding. Thus, reduction of the manufacturing cost cannot be achieved by decreasing the thickness of the chip.